The present invention generally relates to an image reproducing apparatus. More particularly, the present invention relates to an image reproducing apparatus of the type in which an image reader separates an image of an original into a plurality of color components, and reads the image data of the plurality of color components, the image data of the color components are converted into the record image data of each record color, and the color recording units are operated in accordance with the record image data, to reproduce a color image of the original image.
Recently, a digital copying machine of the electrophotography type in which an original is scanned by deflecting a laser beam modulated by an image signal, has widely been marketed. There are other types of digital copying machines, such as ink jet type copying machines and the thermal transfer type copying machines. Of those types of digital copying machines, the digital copying machine of the electrophotography type is advantageous in that a copying speed can readily be increased and the running cost is low.
This type of the digital copying machine is disadvantageous in picture quality. This is due to the structure of the fixing stage. Particularly in the color image, the picture quality problem is remarkable. Generally, a color image is formed by mixing toners of three colors, yellow, magenta and cyan in a subtractive mixture method. This brings about many disadvantages. For example, The toner layer of the resultant color image is thicker than that of the monochromatic image. Toner fusing heat as required is high. A limited power distribution can provide unsatisfactory printed or reproduced pictures. More exactly, the flatness and gloss of the printed picture are poor.
A color image formation is also known, in which a color image is once formed on an overhead projector sheet (OHP) of resin, and the light transmitted through the OHP is used for the image formation. In this type of the color image formation, the requirements for the toner fusing and the flatness of the printed image surface are more strict. The color reproduction by the reflected light is satisfactory, but that by the transmitted light provides a gloomy picture. In the extreme case, the picture quality is below a practical level.
The above problems have been pointed out by Japanese Patent Application Laying-Open (KOKAI) No. 60-211483 which proposes to decrease the speed of rotation of the fixing heat roller in forming an image on the OHP sheet. The color copying machines designed on the basis of the proposal have bee developed and marketed.
In the copying machine of this type that practically used, the speed of rotation of the fixing heat roller is indeed reduced, but the process linear velocity in the developing and transfer stages cannot be changed. Accordingly, if the paper still under transfer process enters the fixing rollers, two different velocities are applied to the paper, the paper becomes less tense, and the unfixed toner possibly rubs on the mechanical part. To cope with this, the paper transport path is designed to have a satisfactory distance between the image transfer point and the fixing unit. This approach, however, makes the copying machine bulky. A measure taken for the bulky problem by some copying machines is to limit the OHP sheet to small size sheet, e.g., A4 size. In this sense, the measure succeeds in holding back the elongation of the transport path. Another copying machine has been proposed in which the the process linear speed of the development and transfer is changed and set at a desired linear velocity of the fixing rollers. In other words, the copying machine is designed to operated at two different copying speeds. The proposal has smoothly be introduced into the digital copying machines other than that of the laser beam scanning type. So far as we know, the digital copying machine based on the proposal has not been developed yet. Japanese Patent Application Laying-Open (KOKAI) No. 57-104102 discloses a technique using a plurality of rotating mirror scanners in which the mirror scanners are appropriately switched to change a scanning speed of the laser beam. The technique obviously increases the cost to manufacture, because of using many expensive mirror scanners. Some specific numbers of mirror surfaces make it difficult to design lens. In the case of the polyhedron mirror, a displacement of the deflecting point, called a sag, depends on the number of surfaces of the mirror. The displacement affects a curvature of the image surface in the sub-scanning direction. Accordingly, when the number of the mirror surfaces and the mirror size are selected to be some specific values, designed tolerances of the mirror cannot be satisfied.
The scanning speed in the main scanning direction is also changed. This requires change of the picture element clock frequency and the optical conditions.